This invention relates to a process for preparing electroluminescent thin films, and more particularly to a multi-source reactive deposition process for preparing a blue light emitting thin phosphor film.
Blue light emitting phosphors suitable for use in full-color thin film electroluminescent flat panel displays have previously been identified. U.S. patent application Ser. No. 07/668,201 to Sun et al. filed Mar. 12, 1991, now U.S. Pat. No. 5,309,070 discloses a blue light emitting phosphor film, CaGa.sub.2 S.sub.4 :Ce, prepared by a sputter-deposition technique. However, the deposited film is an amorphous film and therefore a post-deposition high temperature thermal anneal is required to crystalize the deposited film. Only a costly high temperature glass-ceramic substrate can maintain dimensional stability under the demanding annealing conditions. Thus, the sputter-deposition technique is not compatible with a less expensive borosilicate glass substrate, the conventional flat panel display substrate.
The crystalline structure of a deposited film is affected by the substrate temperature, and a relatively high substrate temperature is desirable. However, using a higher substrate temperature results in a film deficient in certain elements when the film is deposited from a single source. Thus, M.sup.II Ga.sub.2 S.sub.4 :Ce films formed by sputter deposition using a single target at substrate temperatures higher than about 300.degree. C. are deficient in gallium and sulfur.
A low temperature evaporation technique for the preparation of either a Eu- or EuCl.sub.3 -doped CaS phosphor is disclosed by Mauch et al. in "Preparation of Eu- and EuCl.sub.3 -Doped CaS Based Thin Film Electroluminescent Devices," Acta Polytechnica Scandinavica, Applied Physics Series 170, 227 (1990). The doped phosphor layer is deposited by coevaporation of calcium, sulfur and dopants from graphite based effusion sources. T. Nire et al., in "Multi-Source Deposition Methods for ZnS and SrS Thin-Film Electroluminescent Devices," Electroluminescence, Springer Proceedings in Physics, Vol. 38, 180 (1989), disclose a multi-source deposition method for the deposition of the blue light emitting phosphor SrS:Ce. The phosphor layer, SrS:Ce, was fabricated by using three sources: Sr, S, Ce or Sr, S, CeCl.sub.3 ; and four sources: Sr, S, Ce, KCl or Sr, S, Ce, K.sub.2 S. However, an attempt to deposit a film of SrGa.sub.2 S.sub.4 :Ce in a multi-source reactive deposition process using Sr, Ga, S and CeCl.sub.3 sources was unsuccessful because excess gallium segregates into droplets on the film which results in poor reliability of the finished electroluminescent device.
What is still needed is an improved process for the preparation of a crystalline blue light emitting phosphor layer for an AC TFEL device on conventional flat panel borosilicate glass substrates.